222 Intelligence and Miscellaneous Articles. 



PRINCIPLE OF RELATIVITY. 

 To the Editors of the Philosophical Magazine. 

 Gentlemen, — Antibes, May 16th, 1911. 



Mr. E. C. Tolman, in an article which appeared in the Philo- 

 sophical Magazine (vol. xxi. March 1911), points out how the 

 principle of relativity conduces to the expression of the force to 

 which an electric charge in an electromagnetic field is submitted. 



I laid before the Congress of Badiology and Electricity of 

 Brussels (September 1910) the following consequences of the 

 principle of relativity : — 



A charge in uniform motion creates, besides its electrostatic 

 field, an induced field (corresponding to induction by the motion 

 of a current of constant intensity) — and a magnetic field. 



A charge in uniform motion in a magnetic field is subjected, by 

 the fact of its motion, to the force of Hall-Lorentz normal to the 

 speed and to the field, proportional to their product and to the 

 sine of their angle, a result Mr. Tolman has just established. 



The volume of the Communications of the Congress has not yet 

 appeared, but a summary of my results is inserted in Le Radium 

 (August number, 1910). 



I should be very much obliged to you, Gentlemen, if you would 

 be good enough to accord my letter a place in the next number of 

 the Philosophical Magazine. 



Believe me, 



Yours faithfully, 



E. M. Lemeeay. 



THE MODE OF IONIZATION BY X-RAYS. 



The University, Leeds. 

 June 6, 1911. 



To the Editors of the Philosophical Magazine. 

 Gentlemen, — 



In the June number of your Magazine Prof. Millikan and 

 Mr. Eletcher state (p. 761) that an oil drop which they were 

 observing caught very few negative ions when placed a few 

 millimetres (as I judge from their diagram) to one side of a stream 

 of X-rays passing through air at atmospheric pressure ; the im- 

 posed electric field being such as to urge negative ions made in the 

 stream to move away from the drop. This, the authors say, " shows 

 conclusively that the greater part of the ionization of a gas by 

 X-rays is due to the direct action of the primary rays/' 



The experiment does indeed show that very little of the ionization 

 was due to secondary rays which penetrated the air so far from 

 the primary stream as to get at the other side of the drop : this 

 was to be expected. But it does not clash at all with a certain 

 consequence * of the corpuscular theory of X-rays, which is, that 



* Bragg, " Consequences of the corpuscular hypothesis of the y and 

 X-rays," Phil. Mag. Sept. 1910. 



